Traditionally wireless Radio Frequency (RF) Assistive Listening Devices (ALD) have utilized narrow band analogue Frequency Modulation (FM) as the wireless technology for aiding one or more listeners in receiving an audio signal from an audio source (e.g. a talker/speaker). A wireless ALD system consists for example of a microphone used by a speaker for picking up a sound signal from the speaker and comprising a transmitter for wirelessly transmitting the acoustic signal and a receiver used by a listener for receiving the wireless signal. The receiver is adapted to transmit the sound signal to a part of a listening device (e.g. of a hearing aid) worn at or in an ear of the listener/user, e.g. either through a direct audio input to the ear worn part or through a wireless link to the ear worn part, e.g. provided using a looped cord worn around the neck of the user and a corresponding inductive receiver in the ear worn part of the listening device.
Wireless ALD systems are useful in a variety of situations or venues such as theatres, places of worship, museums, public meeting places, corporate conference rooms, convention centres, other large areas for gathering listening, during classroom lectures, in a restaurant, at meetings, at hospitals, senior centres etc.
Narrow band analogue FM based ALD systems have several limitations, however, some of which are    1. Limited audio dynamic range, due to bandwidth limits in the RF channel and thereby limitation in the FM frequency deviation—maximum audio SNR is approximately 50 dB.    2. Limited number of RF channels for high density areas—limited RF spectrum for wireless ALDs in some countries. The available spectrum is different in almost every country and the amount of spectrum is usually limited to between 0.2 MHz and 2 MHz. The available frequencies are clustered around 169 MHz (Japan, Denmark), 174-176 MHz (most of Europe, S. Africa, Brazil, Hong Kong, Australia), 180-186 MHz (Denmark, France, Belgium, Singapore) and 216-217 MHz (USA, Canada, Taiwan, Korea). Also new frequencies (169.4-169.6 MHz) are in the process of being released for exclusive use in European countries.    3. Limited distance or range between transmitter and receiver—due to limits in maximum allowed RF Transmit (TX) power.
Typical maximum allowable transmission power levels for three analogue narrow band FM ALD frequency bands are shown in the table below:
BandLevel173 MHz 2 mW ∝  3 dBm radiated183 MHz 10 mW ∝ 10 dBm radiated216 MHz100 mW ∝ 20 dBm radiated
To overcome the limited audio quality of maximum 50 dB SNR in analogue narrow band FM systems, a shift to a digital modulation based system could be considered. Unfortunately the use of digital modulation schemes is not free of problems e.g.:    1. A lack of dedicated universal or close to universal frequency bands for hearing impaired ALD applications utilizing digital modulation schemes.    2. Limits on TX power before frequency hopping must be implemented.    3. Has to coexist with other unknown systems/applications in the same frequency band/spectrum.
U.S. Pat. No. 6,397,037 B1 describes an RF-type amplification system, such as a classroom amplification system, employing technology (e.g. digital technology) adapted from a cordless telephone system to overcome interference. WO 2006/074692 A1 describes a hearing aid comprising an ear-piece unit to be placed in or at a user's ear, the ear piece comprising a microphone unit and a loudspeaker unit and a wireless interface, the hearing aid further comprising a body worn amplifying device adapted to amplify electrical signals received from the microphone unit to deliver the amplified electrical signals to the loudspeaker unit. The wireless interface can e.g. be based on Bluetooth, DECT or other digital standards. WO 2009/118424 A2 describes a hearing assistance system comprising a transmission unit comprising a microphone arrangement for capturing audio signals from a speaker's voice and means for transmitting audio signals via a wireless audio link; at least one receiver unit for receiving audio signals from the transmission unit via the wireless audio link, the receiver unit comprising a speech quality indicator unit comprising means for assessing the speech quality of the received audio signals and means for providing a signal representative of the assessed speech quality; means for stimulating a user's hearing according to the audio signals received by the receiver unit; and means for providing an alarm signal if the assessed speech quality is below a predefined threshold value. The alarm signal is e.g. transmitted from the receiver unit to the transmitter unit via a (separate) assistive digital link.
An ALD application based for example on Bluetooth transmission, e.g. used in a public address system, e.g. a classroom amplification system, is not feasible because it is peer to peer based.
Further, in body worn listening devices comprising a local source of energy (e.g. a battery) and in particular in ear worn devices due to their small size (and corresponding small space for a power source), low power consumption is a major issue.